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Lung cancer stages indicate whether and how much cancer has spread, explains WebMD. Staging is determined by tumor size and the presence or absence of cancer cells in the lymph nodes or other body organs. Lung cancer stages are determined by x-rays, bone scans, PET scans and CT scans. Blood tests detect metastasized lung cancer cells in the liver or bones. Staging determines which, if any, treatments are used.

Stage I lung cancer is limited to one or both lungs. In stages II and III, cancer cells may appear in the lymph nodes. Lung cancers that spread outside the lungs are classified stage IV, according to WebMD.

Lung cancer is the second-most diagnosed cancer in women and men, states WebMD. Lung cancer is the number one cause of cancer deaths. Approximately 160,000 people die of lung cancer every year. Approximately 85 percent of lung cancer cases are caused by cigarette smoking. Smoking one pack of cigarettes per day increases a person's likelihood of developing lung cancer 20 times. Persons who smoke two packs a day triple their risk of developing lung cancer.